The present invention relates to a network apparatus which recognizes interconnection provided therebetween by holding a cable at a specified intermediate potential such as those used for an interface in accordance with, e.g., the IEEE Standard 1394. More particularly, the present invention relates to a technology for compensating for operation when a power supply is interrupted.
As a conventional network apparatus, there has been known a structure which recognizes the level of an intermediate potential supplied from another network apparatus connected thereto via a first cable and supplies an intermediate potential to a second cable such that the other network apparatus recognizes the level of the supplied intermediate potential, whereby the interconnection provided between the network apparatus is recognized (see, e.g., Japanese Patent Laid-Open Publication No. Hei 11-161444). In the network apparatus, if one of the interconnected network apparatus recognizes a halt in the supply of the intermediate potential from the other, the network apparatus disconnects itself from the other. If a power supply from the other network apparatus is initiated again, the network apparatus recognizes the connection again.
As a result of conducting studies, however, the present inventor has found that the conventional apparatus had the following problems.
The network apparatus is internally provided with a plurality of power supplies so that, in response to a control signal generated by one of the power supplies, an intermediate potential generated by another is supplied. In this case, if a power supply to the entire apparatus is interrupted, the control signal may become unsteady depending on potential reductions at the individual power sources irrespective of normal operation still performed by an intermediate potential supply unit.
Consequently, a current flows to the power supply for the intermediate potential supply unit via a cable through which the intermediate potential is supplied from another network apparatus connected to the network apparatus of concern so that additional power is supplied from the power supply to a driver supplying the intermediate potential. This causes the situation in which the level of the intermediate potential which should be lowered to the ground potential in principle is not lowered completely due to the additional power supplied from the other network apparatus.
As a result, the other network apparatus recognizes by mistake that the connection is still maintained since the level of the intermediate potential on the cable is higher than a specified value. In other words, the other network apparatus retains the recognition that the connected state is still maintained irrespective of the interruption of the power supply to the network apparatus of concern.
The other network apparatus respondingly tries an access to the network apparatus disconnected from the power supply by judging that the network apparatus is still active but the access cannot be made actually. This causes the problem that the network system is brought into an abnormal state.